1. Field of the Invention
The invention generally relates to apparatus and method for reducing interference in modulated signals received in a communications systems, and in particular to complementary code keying modulation techniques in WLAN (Wireless Local Area Network) systems.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections. Thus, wireless LANs combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread spectrum technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows the speed increase.
The CCK modulation can generally be described as a modification of MOK (M-ary Orthogonal Keying) modulation using codes of complex symbol structure. The CCK technology allows for multi-channel operation and employs the same chip rate and spectrum shape as the 802.11 Barker code spread functions. CCK can be considered as a form of M-ary code word modulation where one of M unique signal codewords is chose for transmission.
Referring now to FIG. 1 which illustrates a block diagram of a conventional CCK modulator, a multiplexer 100 receives scrambled data and multiplexes the input data either to the code selector 110 or to the modulator 120. The multiplexer 100 gets clocked at the symbol rate. The code selector 110 selects one of 64 complex codes to be fed to the modulator 120. The bits that the modulator 120 receives from the multiplexer 100 are used to QPSK (Quadrature Phase Shift Keying) modulate the codeword. The outputs of the modulator 120 are I and Q outputs to generate complex codes.
In particular the 802.11b CCK-11 (Complementary Code Keying at 11 Mbps) modulation is subject to multipath propagation. While (direct sequence) spread spectrum systems are usually designed to cope with multipath propagation, the spreading gain for CCK-11 modulation is only two so that this mode degrades considerably more than all other 802.11b modes in a multipath environment, if transmission is distorted by frequency selective fading.